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Quorum Response: A Form of Information Cascade Used in Animal Groups

http://rstb.royalsocietypublishing.org/content/364/1518/743.full.pdf

So far in class, we have learned that information cascades can produce wrong choices, be based on very little private information, and are fragile. The primary lesson learned from information cascades, as mentioned in the course textbook, is to be careful drawing conclusions about the behaviors of crowds, as the crowd can be wrong, even if everyone is rational and have the same goal in mind. However, information cascades are not necessarily bad. Previous research on decision-making behaviors in animal groups have demonstrated that information cascades provide animals with a fast and efficient way of reaching a consensus. This form of information cascade is called “Quorum Response”. A quorum response, as defined by Sumpter & Pratt in their 2009 research paper (see link), is a nonlinear function in which the probability an animal is going to display a certain behavior is dependent on the number of other individuals already showing that behavior. In other words, an animal is more likely to commit to a certain option if there are other animals already committed to that option.

Based on that definition, a quorum response sounds very much like an information cascade, but the two are quite different. Information cascades often result in wrong decisions, because after a while, individuals start paying less attention to their own private information and more attention to public information, leading to a cascade of uninformed choices. One way to solve this problem is to let individuals independently gather information on their available options and then compare their findings to other individuals’ findings. This is how animal groups reach a consensus. Unlike human societies, animal societies are often decentralized, with no specific leader telling everyone what to do or integrating all the information. As such, animals are often required to gather their own information and share it with other individuals through a series of local interactions. As more information is passed through the group, individuals will eventually realize that one particular option is more favorable than the other, because more individuals are sharing information about that option. As more individuals start favoring that one option, the probability that a single individual will select that option increases. Eventually, when a threshold of individuals favoring that option is reached, the entire group will conform towards that option, resulting in a consensus.

From this, we can see how quorum responses can provide groups with the speed and cohesiveness of an information cascade but with a higher group accuracy often associated with independent voting. This makes sense, because as more individuals are contributing their private information to the group and voting independently (or partially independently), the chances of making a well-informed and correct decision increases. However, that does not mean quorum responses give groups the right choices all the time. Like information cascades, quorum responses can produce wrong choices. A good example of this provided in Sumpter & Pratt’s paper is quorum response in fish. When a school of fish must decide between two routes, they first look to see how many conspecifics are already moving towards a specific route. The higher the number of conspecifics already moving towards the route, the more likely that school of fish will take that route, but Sumpter & Pratt noted that a school of fish will still move towards that route even if a predator was present in that direction.

Despite the possibility of making wrong decisions following quorum responses, many group-living animals and insects, such as honeybees, ants, birds, and primates, still use quorum responses when making group decisions. Quorum responses are very similar to information cascades in terms of quickly spreading information over a large network, but it is much more accurate and reliable than information cascades, because individuals utilize both private and public information.

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